Extended source E-beam mask imaging system and method

Abstract

An electron beam imaging system (10) includes a photoemitter plate (12). An optical image beam (15) is directed through a pattern mask (18), which is imaged onto the photoemitter (12). The photoemitter (12) emits electrons from those unmasked regions illuminated by the optical image beam, emitting an extended-source electron beam that carries the mask image. The extended-source electron beam is focused (34) onto a device under fabrication (40), providing a single-stage electron lithographic patterning function. The optical source (16) is chosen so that the optical image beam energy is nearly identical to the work function for the photoemissive coating (14) of the photoemitter (12). As a result, the photoemitter (12) emits electrons with substantially zero kinetic energy, allowing the emitted electrons to be accelerated through the electron beam focusing elements (34) with very nearly identical electron velocities, thereby minimizing chromatic aberrations. In one embodiment, an aperture (85) is used to limit the extended-source electron beam to those electrons with trajectories requiring no more than a maximum amount of focusing, thereby minimizing spherical aberrations.

What is claimed is: 
     
       1. A method of imaging a mask pattern on a device under fabrication using electron lithographic patterning, comprising the steps: directing an optical image beam through a mask defining a mask pattern, which in turn creates an unmasked image beam, onto a photoemissive source, wherein said photoemissive source is provided for emitting electrons in response to said image beam;   in response to said image beam, said photoemissive source generating an extended-source electron beam defined by electrons emitted from those regions of said photoemissive source on which the unmasked image beam is incident;   focusing said extended-source electron beam onto the device under fabrication such that the mask pattern is imaged onto the device; and   accelerating said extended-source electron beam away from said photoemissive source such that the electrons in said extended-source electron beam maintain substantially identical velocities during the focusing step.   
     
     
       2. The method of claim 1, wherein the energy of said optical image beam is substantially identical to the work function of said photoemissive source. 
     
     
       3. The method of claim 2, wherein: the image beam comprises light from an Nd:YAG laser frequency-quadrupled to provide a wavelength of about 266 nanometers; and   said photoemissive source comprises gold.   
     
     
       4. The method of claim 1, further comprising the step of transmitting said extended-source electron beam emitted from said photoemissive source through an aperture element that blocks electrons emitted with trajectories requiring more than a selected maximum amount of focusing. 
     
     
       5. The method of claim 2, further comprising the step of transmitting said extended-source electron beam emitted from said photoemissive source through an aperture element that blocks electrons emitted with trajectories requiring more than a selected maximum amount of focusing. 
     
     
       6. A method for patterning a device under fabrication comprising: providing a light source for generating an optical image beam;   providing a photolithographic mask that defines a pattern to be used in fabricating the device;   providing a substantially planar photoemissive source responsive to the image beam from said light source for emitting electrons;   directing said optical image beam through said mask onto said photoemissive source, thereby imaging onto said photoemissive source the pattern defined by said mask;   said photoemissive source emitting a spatially extended electron beam carrying the mask-image defined by the unmasked portions of the incident image beam; and   focusing said extended-source electron beam onto the device thereby providing a pattern on the device corresponding to the mask-image carried by said extended-source electron beam.   
     
     
       7. The method of claim 6, wherein said light source generates an optical image beam with photon energy substantially identical to the work function of said photoemissive source. 
     
     
       8. The method of claim 7, further including the step of accelerating electrons emitted from said photoemissive source away from said source and through said electron beam focusing means with substantially identical electron velocities. 
     
     
       9. The method of claim 8, wherein: said light source comprises an Nd:YAG laser source and a frequency-quadrupling element, thereby providing coherent light with a wavelength of about 266 nanometers; and   said photoemissive source comprises gold.   
     
     
       10. The method of claim 6, wherein said focusing step includes using an aperture having a central hole for blocking electrons with trajectories that require more than a selected maximum focusing angle. 
     
     
       11. The method defined in claim 10, further including the step of accelerating emitted electrons at a nearly uniform rate of acceleration across the extended electron source.